1. Field of the Invention
This invention relates to niobium or tantalum powder suitable as an electrolyte capacitor material, and to a method of manufacturing the niobium or tantalum powder, and more particularly to a manufacturing method that reduces the niobium or tantalum oxides with alkali metals or alkaline-earth metals.
2. Description of Related Art
By reducing fluorides or oxides of niobium or tantalum with alkali metals such as sodium or potassium, or with alkaline-earth metals such as magnesium or calcium, and then evenly distributing the particles having an average diameter of 1.0 μm, it is possible to obtain very fine niobium or tantalum powder having a BET specific surface area that exceeds 5 m2/g.
When using niobium or tantalum powder as electrolytic capacitor material, the capacity becomes larger the finer the powder is, or in other words the larger the specific surface area is.
However, niobium oxidizes easily, and since the oxides are stable, the finer the powder is, the greater the amount of oxygen becomes due to surface oxidation.
Moreover, when trying to obtain fine powder, in which the diameter of primary particles is 0.1 to 1.0 μm, by using alkali metals or alkaline-earth metals as the reduction agent, which is an exothermic reaction, there is thermal ununiformity during the reduction process, so very fine particles on the order of 0.01 μm are produced. When these kinds of very fine particles exist, there is the problem that uniformity in sintering of the electrolytic capacitor is lost.
Furthermore, there is a problem that the reduction agent or alkali metal or alkaline-earth metal dissolves and remains in the niobium or tantalum and has an adverse effect on the capacity and electrical characteristics of the electrolytic capacitor. In other words, the amount of reduction agent used in normal operation is more than the theoretical chemical equivalent, so part of the reduction agent dissolves in the niobium or tantalum. The solid solubility limit of alkali metals or alkaline-earth metals in the niobium or tantalum is small, however, even so alkali metals or alkaline-earth metals remains up to about 200-400 ppm. Therefore, with the prior art, it was difficult to reduce the amount of the reduction agent, or in other words, alkali metal or alkaline-earth metal to 200 ppm or less.
On the other hand, reduction of the niobium or tantalum oxides with alkali metal or alkaline-earth metal proceeds exothermically and the alkali metal or alkaline-earth metal is processed to come in direct contact with the niobium or tantalum oxides, so that controlling it becomes even more difficult.
In Japanese Patent Publication No. Tokukai 2000-119710, in order to control the amount of heat generated, reduction using alkaline-earth metal or rare-earth metal is performed in two stages, and in the reaction of the first stage, low-grade oxide powder expressed as (Nb,Ta) Ox, where x=0.5 to 1.5, is obtained. The second stage is then performed alter removing the oxides of the reduction agent from the reduction product of the first stage. However, in this case, controlling the reduction reaction of the second stage is difficult and there was a problem in that the specific surface area of the powder obtained became small, and that the amount of remaining alkali metal and alkaline-earth metal became large.